The present invention relates to a method for manufacturing a magnetic recording medium. In particular, the present invention relates to a method for manufacturing a magnetic recording medium applied to a perpendicular magnetic recording technology.
Attention has been paid to perpendicular magnetic recording system as a technology for improving the areal recording density to achieve an increase in the capacity of a magnetic recording device. A perpendicular magnetic recording system is a means to form the recording bits in which magnetization of the recording medium is made normal to the medium face and the magnetization in the adjacent recording bits is made anti-parallel to each other. In a perpendicular magnetic recording system, since the diamagnetic field in the magnetic transition region is small, a sharp magnetic transition region is created compared with a longitudinal magnetic recording system, resulting in the magnetization being stabilized at a high density. Therefore, compared with a longitudinal magnetic recording system, it is possible to make the film thickness greater and the volume of the ferromagnetic grains greater to obtain the same resolution, thus it is possible to control the time-varying decay of the recorded magnetization, that is, the thermal demagnetization. Moreover, when it is combined with a perpendicular magnetic recording medium which has a single pole type head, a perpendicular recording layer, and a soft magnetic underlayer, a high recording magnetic field can be obtained and selecting a material having a high magnetic anisotropy for the perpendicular recording layer becomes possible, so that the thermal demagnetization can be further controlled.
In these days, a CoCr-based alloy crystalline film has been the mainstream for a material of a recording layer of a perpendicular magnetic recording medium. The easy axis of the recording layer can be maintained perpendicular by controlling the crystallographic texture to make the c-axis of a CoCr crystal having a hexagonal close-packed (hcp) structure normal to the medium face. Herein, the media noise can be reduced by making the grain size of the CoCr-based crystal smaller, decreasing the distribution, and reducing the inter-granular magnetic exchange interaction, thereby, the recording density can be improved. As a means to control such a recording layer structure, a recording layer, in general a so-called granular thin film, is proposed in which ferromagnetic grains are surrounded by a non-magnetic material such as an oxide. In a granular recording layer, a non-magnetic grain boundary phase separates the magnetic grains, the inter-granular exchange coupling is decreased, and the noise at the magnetic transition region can be reduced. JP-A No. 178413/2003 discloses a perpendicular magnetic recording medium which has a recording layer composed of a ferromagnetic alloy containing Co and Pt and an oxide having a volume density from 15% to 40%. Moreover, in IEEE Transactions on Magnetics, Vol. 40, No. 4, July 2004, pp. 2498-2500, “Role of Oxygen Incorporation in Co—Cr—Pt—Si—O Perpendicular Magnetic Recording Media”, a means was disclosed for depositing a recording layer which has a granular structure by a DC magnetron sputtering method in an argon-oxygen mixed gas atmosphere using a complex target containing a CoCrPt alloy and SiO2. It is reported that the coercivity is increased and the read/write characteristics is improved by doing a reactive sputtering in an atmosphere including oxygen.
Moreover, in a magnetic recording medium for a longitudinal recording system which has been used conventionally, a magnetic recording medium having a granular structured recording layer has been disclosed in JP-A No. 178423/2003.